The electrostatic spraying of liquids and powders for the coating of articles, such as spray painting and the like is well known, and it is only recently that this has been extended to other areas.
Australian Patent Specification No. 475,120 is directed to the spraying from an airborne source, such as an aeroplane where a primary and secondary spray of material are concurrently emitted, both sprays being electrostatically charged of opposite polarity to preclude intermixing of the two sprays before the sprays are deposited. This specification discloses that the liquid is first atomized and then charged, or a liquid is vapourised, the vapour saturating a moving air flow then passes through an electrogasdynamic type channel to produce extremely small sized charged particles.
Australian Patent Specification No. 26794/77 discloses the spraying of pesticides in which a spray head has a conducting or semiconducting surface to which a potential is applied, a field intensifying electrode being mounted adjacent the surface which electrode is earthed, so that the electrostatic field causes liquid to atomize to form electrically charged particles to be projected past or away from the electrode.
Australian Patent Specification No. 63814/80 is directed to a rotating spray head with a circular induction electrode disposed adjacent to the spray head rim, the spray head rotating to produce the desired spray from the spray head.
Australian Patent Specification No. 79851/82 discloses an agricultural sprayer, there being provided means to produce a descending cloud of charged pesticide particles and further elements being electrically connected to a potential spaced from the spray head so that there is little or no tendency for the charged particles to contaminate the operator.
The spraying devices in the agricultural field thus utilize electrodes which atomize the sprayed particles, and these electrodes are often exposed from the spray head, and various other units rely on the coronal discharge for charging the particles.
U.S. Pat. Nos. 4,073,712, 4,073,273 and 4,199,430 disclose the application of an electrostatic field to a flow of liquid through a cylinder.
U.S. Pat. No. 2,975,757 shows electrostatic spraying by charging the sprayed material by impinging the droplets on a rotating charged element, so that the droplets are charged as they impinge on the element and are thrown therefrom.
U.S. Pat. No. 2,899,136 is directed to electrostatic paint spraying apparatus where a centrifugal distributor is charged, the paint being fed to the distributor to be charged and distributed therefrom.
U.S. Pat. No. 3,031,145 discloses an electrostatic paint spraying apparatus, wherein a conventional spray gun is charged, the paint being charged in a pool as it passes to and through the gun.
U.S. Pat. No. 3,117,029 discloses an electrostatic coating apparatus wherein an electrical potential between the spray nozzle and the object being coated provided.
U.S. Pat. No. 3,127,112 discloses the spraying of a paint and a solvent, the solvent spray head having a potential applied thereto to provide a potential difference between the solvent spray head and the object being coated, the solvent being sprayed into the paint spray emitter from the paint spray head.
U.S. Pat. No. 4,185,783 discloses the spraying of a powder, an electrical potential being applied to the powder in a supply hose to the spray gun to prevent clumps of powder adhering to the supply hose.